Blisters in fibreglass are small, yet potentially serious, problems. These flaws must be avoided when working with fibreglass and fixed if they occur. Ignoring blisters in fibreglass can, at best, be unattractive and, at worst, result in the destruction of your project.
Causes of Blisters
Most fibreglass is a composite material made up of layers of resin and fibreglass cloth or mat. As each layer of fibreglass is applied, it is soaked with resin to secure it in place. If the job is done too quickly or too sloppily, air bubbles can become trapped between the layers. These air bubbles become blisters once the resin has cured.
Why They're Bad
The biggest reason that blisters in fibreglass are undesirable is that they weaken the finished product. Each blister is a thin spot in the fibreglass, allowing for faster cracks and breaks over time. The more blisters that form in fibreglass, the faster the material will break down. Blisters are also a major aesthetic problem in boats, cars and other surfaces that must have a mirror-smooth finish. The slightest lump caused by a blister can catch light and ruin a finish.
When applying fibreglass, it is important to cut it into small pieces to make the removal of air bubbles manageable. If a project is covered with a single sheet of fibreglass, air bubbles at the centre will be difficult to remove. By overlapping small pieces of fibreglass, trapped air bubbles only have to be moved a few inches in order to escape. The choice of fibreglass is important too. Fibreglass mat is thinner and weaker than cloth but is easier to work air bubbles out of.
If you have a blister in cured fibreglass, it must be removed. Begin by drilling out the blister to expose it to air. The hole should be the same size as a small blister, or several small holes should be drilled into a large one. The blister can then be filled with fibreglass resin mixed with a thickening agent, such as fumed silica, to form a paste. Ideally, this paste should be injected into the blister and drilled hole. Once it has cured, the surface can be sanded smooth.